I've replaced my old m.2 256GB Samsung XP941 SSD, my boot drive with Windows 10 installed, with a 1TB 970 EVO Plus. This is on an MSI X99S SLI Plus motherboard that was recently upgraded to the latest non-beta version.
Before I started, I used Macrium Reflect to create a full image of the SSD drive (3 partitions) on my larger non-SSD drive, and created a rescue USB stick. I then replaced the drive physically. After booting with the USB stick, I could restore the image to the new drive successfully.
In the BIOS, Boot Mode is set to Legacy + UEFI as for UEFI only, the new drive does not show. I've set the drive as first boot option. Now when I try to boot from it, Windows gives me blue screen with stop code "Inaccessible Boot Device". After a few times it goes into Automatic Repair mode but this does not help.
Note that in the BIOS settings, under System status, it says "Not present" for the entry M.2/SATA Port 5, although there is definitely an M.2 drive present, as I was able to restore the Macrium image onto it.